1. Field of Invention
The invention relates to a micro electromechanical differential actuator used in micro electromechanical systems (MEMS) such as optical switches, variable optical attenuators, optical tunable filters, modulators, tunable VCSEL""s, grating modulators, micro displays, optical information and RF switches. In particular, the invention pertains to a micro electromechanical differential actuator that has low optical/RF loss and micro mirror stabilization operation.
2. Related Art
Optical actuators made using the micro electromechanical system (MEMS) technology can be applied to optical switches, variable optical attenuators, optical tunable filters, modulators, tunable VCSEL""s, grating modulators, and RF switches. In these devices, how to manufacture a low optical loss mirror and how to make no-tilt-angle parallel motion stabilization operations are the key techniques of great importance.
With reference to FIG. 1A, in the conventional RF switch technique, the film layer 11 (upper electrode) is at its upper position when it is in the OFF state. When the RF switch is in the ON state, the film layer 11 is dragged by an electrostatic force to touch the lower dielectric material layer 12 (the lower electrode layer 13) to achieve the RF switch action. The disadvantage of this type of switches is the structural deformation shown in FIG. 1B. This results in the effect that the contact area grows during the transient state from a point to an area. The contact force during the area growth also varies, as shown in FIG. 1C. This is the well-known problem of xe2x80x9ccontact area reliabilityxe2x80x9d in RF switches. One therefore sees how important it is to develop a planar actuator that makes parallel motions to conquer the reliability problem.
As shown in FIGS. 2A and 2B, a micro optical electromechanical tunable VCSEL of prior art (Bandwidth 9) has a distributed Bragg reflector (DBR) mirror, which utilizes a micro electromechanical suspension arm 15. The upper layer material of the suspension arm 15 is GaAs 151 with a larger coefficient of thermal expansion. The lower layer material of the suspension arm is a DBR layer 152, 153 with a smaller coefficient of thermal expansion. During the operation, the p-DBR layer 153 uses an electrostatic force to attract the n-DBR layer 152, producing a downward motion to modulate the outer cavity wavelength. However, when the electrostatic field is imposed, the mode limitation of the suspension structure produces a tilt angle at the terminal of the suspension arm. Thus, the n-DBR layer 152 can not be maintained horizontal. The net effect is to produce a laser beam with shifted and asymmetric beam profile outputs, increasing the coupled optical loss in optic fibers.
As shown in FIG. 3, the micro optical electromechanical modulator of prior art (Lucent) uses an in-plane center-symmetric four-string structure 17 to achieve the vertical parallel motions of the support mirror. However, due to the close rigidities of the support mirror as well as the suspension arm and the property of the structural influence line, a deformation curve similar to those in FIGS. 1B and 1C will occur. This reduces the optical clear aperture and, therefore, the center-symmetric support mirror deforms in response to the strain to produce a larger optical loss.
With reference to FIGS. 4A and 4B, the conventional micro optical electromechanical grating modulator of prior art (Silicon Light Machine) is fixed by its two ends onto a symmetric bridge structure in the middle of a substrate 16 for the support mirror to make vertical parallel motions. However, due to the close rigidities of the support mirror and the suspension arm as well as the property of the single-layer structural influence line, a deformation curve similar to those in FIGS. 1B and 1C will occur, too. The center-symmetric support mirror deforms in response to the strain and produces a larger optical loss. The grating modulator performs an out-of-plane action, only modulating the diffraction order efficiency. Nevertheless, if one can design an in-plane motion, then the diffraction light can be modulated to achieve the function of in-plane phase shifting, the application including (polarization mode dispersion) PMD compensator and micro interferometer.
The drawback of using the surface micromachining technology to prepare an optical switch with the micro mirror actuator structure is that the mirror thickness is limited between 2 xcexcm and 5 xcexcm with a diameter between 350 xcexcm and 500 xcexcm during the machining process possess with good optical performances. In these conditions, the micro mirror is likely to experience opto-thermal deformation, resulting in a greater optical loss. This is not acceptable for optical communications. Therefore, it is important to design a silicon wafer mirror as a mirror optical switch with a high-quality optical surface and high rigidity so that good optical properties can be maintained in long-term operations.
In view of the foregoing, it is highly desirable to design a micro electromechanical actuator with two degrees of freedom of parallel motion as well as extremely low mirror deformation and optical loss. Such actuators can be applied to such devices as optical switches, variable optical attenuators, optical tunable filters, modulators, tunable VCSEL""s, grating modulators, and RF switches. The ultimate goal is to achieve low loss and stable operations.
The invention provides a micro electromechanical differential actuator, which can make no-tilt-angle motions with two degrees of freedom. That is, the differential actuator is able to make in-plane and out-of-plane vertical and horizontal parallel motions. In addition, the differential actuator design can compensate tilt error or vice versa.
The disclosed micro electromechanical differential actuator uses a single-sided suspension arm set differential design or double-sided bridge suspension arm set differential design. The structural rigidity of the suspension arm set is smaller than that of the support part. The suspension arm set consists of upper, lower, left and right suspension arms, so that the differential suspension arm set absorbs the rotation change when being driven by an external force. Therefore, the support part does not tilt and makes out-of-plane vertical motions and/or in-plane horizontal motions due to its symmetry.
The driving methods of the invention include the electrostatic force, Joule thermal force, electromagnetic actuation, piezoelectric force or other equivalent means.